Two goals of increasing importance in the operation of internal combustion engines are the minimization of exhaust emissions and the improvement of fuel efficiency. These goals can be accomplished, in part, by maintaining an optimum air-to-fuel ratio during combustion. Optimization of the air-to-fuel ratio requires careful measurement of air flow to the engine. Such measurement is often accomplished through the use of a mass airflow sensor.
A mass airflow sensor typically comprises a metallic or plastic housing forming a portion of the air flow chamber between the engine air filter and manifold. Electronic circuitry forming the sensor itself is generally housed in a smaller plastic compartment located atop the metallic or plastic housing. The sensor ordinarily has two elements suspended within the housing such that they are exposed to air flowing therethrough. These elements are the most critical components to the proper operation of the mass airflow sensor electronic circuitry.
In order for the sensor to accurately measure air flow, the sensor elements are subject to strict physical requirements. First, the elements must typically have temperature coefficients of resistance (TCR, i.e., the slope of the line plotting the resistance of the element as a function of temperature) within approximately 1% of each other. Second, deviations in resistance ratios of the elements at selected ambient temperatures must be minimized. Depending on the actual design of the sensor circuitry, to properly minimize such resistance ratio deviations, the elements may be required to have resistances between 19.5 and 20.5 ohms at room temperature, and the difference between the resistances of the elements may be required to be less than 0.1 ohm.
These stringent requirements necessitate "matching" of the sensor elements. In the manufacture of prior art sensors, this matching is accomplished by manually sorting pre-tested elements into groups, where each group contains elements having TCR's and resistance meeting the above described requirements. To have properly matched elements, each sensor must then be constructed using only elements from the same group. These strict requirements necessitate high quality control of pre-tested elements for TCR's and resistance values, and labor intensive efforts for sorting of elements. As a result, prior art sensors are typically expensive to manufacture.
U.S. Pat. No. 4,669,052 issued to Bianco discloses an apparatus and method for calibrating a sensor. Prior art mass airflow sensors typically include circuitry to perform gain and offset adjustments to ensure proper sensor function at room temperature versus the sensing medium. This circuitry is generally implemented through the use of laser trimmed thick film resistors. The invention disclosed in the Bianco patent substitutes a logic unit and database unit for such prior art circuitry to perform gain and offset adjustments, thereby eliminating laser trimming of resistors.
However, the logic and database units of the invention disclosed in the Bianco patent are used to calibrate a sensor being actively tested. As a result, these units become part of the sensor and tend to raise variable production costs. Moreover, the invention disclosed in the Bianco patent does not specifically address the above described problems associated with the "matching" requirements for the sensor elements themselves.
The present invention eliminates the stringent "matching" requirements of prior art mass airflow sensors without sacrificing accuracy of air flow measurement. The present invention also provides for improved air flow measurement accuracy. Each element of a sensor may have TCR's that vary by as much as 6%. Moreover, depending on the actual design of the sensor circuitry, each element may have resistances varying from 18 to 22 ohms at room temperature. As a result, stringent quality control requirements and labor intensive sorting of elements are significantly reduced. The present invention thus dramatically reduces mass airflow sensor manufacturing costs.